1. Field of the Invention
This invention generally relates to an audio apparatus. This invention particularly relates to an audio effector circuit for processing an audio signal to make livelier the music represented by the audio signal.
2. Description of the Related Art
There are various audio effector circuits for processing an audio signal to make livelier the music represented by the audio signal. The audio effector circuits are of a static type and a dynamic type. Examples of the static circuits are a tone control circuit, a graphic equalizer circuit, and a bass boost circuit designed to change the signal amplification gain in a specific frequency range which is fixed or variable. An example of the dynamic circuits is an amplitude expander designed to expand differences between small amplitudes and large amplitudes of an audio signal. Another example of the dynamic circuits is an automatic level control (ALC) circuit having a longer attack time.
For human's sense of hearing, the boost of audio-signal components in a particular frequency range effectively causes the music represented by the resultant audio signal to be livelier.
In the case where an audio signal from a certain music source (for example, a rock-and-roll music source) is processed by a static audio effector circuit such as a tone control circuit or an equalizer circuit which is tuned to the particular frequency range, the music represented by the processing-resultant audio signal can be effectively livelier. On the other hand, in the case where an audio signal from a classical music source is processed by that static audio effector circuit, the music represented by the processing-resultant audio signal tends to be unnatural.
Also, in the case of the processing of an audio signal by a dynamic audio effector circuit such as an amplitude expander, the result of the processing is good or poor depending on the type of a music source of the audio signal. Generally, the processing result tends to be poor when the audio signal comes from a classical music source.
Japanese utility-model application publication number 63-16712/1988 discloses an automatic level control (ALC) circuit in which the level of an output signal from an amplifier is detected, and an input signal to the amplifier is attenuated in response to the detected output-signal level to hold constant the level of the output signal. The ALC circuit in Japanese application 63-16712/1988 includes a charging transistor having a base to which the output signal from the amplifier is applied via a first capacitor and a resistor. A diode is connected between a ground and the base of the charging transistor. The charging transistor controls the charging of a second capacitor connected between the ground and the emitter of the charging transistor. A transistor combination generates a control signal in response to the voltage across the second capacitor. An ALC transistor has a base subjected to the control signal. The ALC transistor attenuates the input signal to the amplifier in response to the control signal.